Chalcogenide materials are an emerging class of commercial electronic materials that exhibit switching, memory, logic, and processing functionality. The basic principles of chalcogenide materials were discovered, and continue to be developed, by S. R. Ovshinsky. The important classes of chalcogenide-based devices include switching devices, memory devices and cognitive devices.
Early work in chalcogenide devices demonstrated electrical switching behavior in which switching from a resistive state to a conductive state was induced upon application of a voltage at or above the threshold voltage of the active chalcogenide material. This effect is the basis of the Ovonic Threshold Switch (OTS) and remains an important practical feature of chalcogenide materials. The OTS provides highly reproducible switching at ultrafast switching speeds for over 1013 cycles.
Another important group of chalcogenide devices are the memory devices. Chalcogenide materials are capable of adopting a crystalline state, an amorphous state and a variety of intermediate structural states with continuously variable proportions of crystalline phase domains and amorphous phase domains in a given volume. The crystalline state is the most conductive state and exhibits the lowest resistance, while the amorphous state is the least conductive state and exhibits the highest resistance. The intermediate states exhibit intermediate resistances that vary with the relative proportion of the crystalline and amorphous phases present. The difference in resistance between the crystalline and amorphous states of a chalcogenide material is commonly several orders of magnitude.
One type of chalcogenide memory device utilizes the wide range of resistance values available for the material as the basis of memory operation. Each resistance value corresponds to a distinct structural state of the chalcogenide material and one or more of the states can be selected and used to define memory states. Each memory state corresponds to a distinct resistance value and each memory resistance value signifies unique informational content. Operationally, the chalcogenide can be programmed into a particular memory state by providing an electric current pulse of appropriate amplitude and duration. Each memory state can be programmed by providing the current pulse characteristic of the state and each state can be read in a non-destructive fashion by measuring the resistance and thus identifying the state. Programming among the different states is fully reversible and the memory devices can be written and read over a virtually unlimited number of cycles to provide robust and reliable operation. The variable resistance memory functionality of chalcogenide materials is currently being exploited in the OUM (Ovonic Universal (or Unified) Memory) devices that are beginning to appear on the market.
Chalcogenide materials also possess a cognitive mode of functionality that serves as another mechanism of memory and that further provides neuron-like properties. In the cognitive mode of operation, a series of structural states is also utilized, but the crystalline phase portion of each of the states is kept at or below the percolation threshold. The states include a series of pre-percolation states, all of which have a relative high resistance, and a set state, which state has a substantially lower resistance than the pre-percolation states and which state corresponds to attainment of the percolation threshold. The available pre-percolation states extend from the amorphous phase of the chalcogenide material to a state having both amorphous and crystalline regions, where the crystalline region is on the verge of crossing the percolation threshold.
Transformations among the pre-percolation states occur by applying energy (for example, electrical energy in the form of current pulses) to the chalcogenide material. Each increment of energy provided to the chalcogenide material induces an increase in the crystalline volume fraction of the chalcogenide material, thereby transforming the material from one pre-percolation state to another pre-percolation state. Since the material lacks a crystalline percolation pathway when it is in its pre-percolation states, the resistance remains high and fairly uniform as the material traverses the different states. Eventually, the crystalline region will have a sufficient volume fraction and will be so situated within the material that percolation is achieved and a contiguous crystalline pathway is formed across the material. At this point, the chalcogenide material sets and a substantial decrease in resistance occurs. Further operation of the material occurs by applying energy in an amount sufficient to reset the material to one of the pre-percolation states.
The energy applied to the chalcogenide material during the transformations between states in the cognitive mode of operation is less than that typically required to transform between the resistance states of the OUM memory. The behavior of the chalcogenide material in the cognitive mode is reminiscent of the biological neuron in that the material accumulates energy as it progresses from one pre-percolation state to the next and fires upon reaching the percolation threshold and transforming to the set state. Each of the pre-percolation states has a structural configuration of crystalline phase material that reflects the net accumulated energy of all energy increments applied to the material since the last reset operation.
The behavior (including switching, memory, accumulation and cognitive operation) and chemical compositions of chalcogenide materials included within the scope of this invention have been described, for example, in the following U.S. Pat. Nos. 6,671,710; 6,714,954; 6,087,674; 5,166,758; 5,296,716; 5,534,711; 5,536,947; 5,596,522; 5,825,046; 5,687,112; 5,912,839; 3,271,591 and 3,530,441, the disclosures of which are hereby incorporated by reference. These references also describe proposed mechanisms that govern the behavior of the chalcogenide materials. The references also describe the structural transformations from the crystalline state to the amorphous state (and vice versa) via a series of partially crystalline states in which the relative proportions of crystalline and amorphous regions vary underlying the operation of electrical and optical chalcogenide materials.
Current commercial development of the chalcogenide materials and devices is oriented toward the fabrication of arrays of devices. Chalcogenide materials offer the promise of high density memory, logic and neural arrays that can operate according to traditional binary data storage or according to a multilevel scheme. Chalcogenide arrays further offer the prospect of integrating, on a single chip, both memory and processing capabilities, thereby enabling high speed operation. The neural functionality provides an opportunity to achieve heretofore unavailable degrees of parallelism as well. Chalcogenide based computational arrays are desirable from a processing point of view since they can be fabricated in an all thin film package that can be readily integrated with conventional silicon devices.
In order to maximize the advantages offered by the chalcogenide materials, it is necessary to form arrays that include a large number of devices, where each device is as small as possible, and, in forming such arrays, to minimize errors in the output response. Strategies for minimizing errors in the reading and writing of data from chalcogenide arrays are desired in the art to further extend the available range of applications.